The application area of chargeable and dischargeable secondary batteries is being increasingly expanded to electric vehicles as well as portable devices such as mobile phones, notebooks, and camcorders. Accordingly, secondary batteries have been actively developed. Also, research and development of battery design to improve capacity density and specific energy have been conducted during the development of the secondary batteries.
In general, it is known that battery safety improves in the order of a liquid electrolyte, a gel polymer electrolyte, and a solid polymer electrolyte, but battery performance decreases in the same order. An electrolyte in a liquid state, particularly, an ion conductive organic liquid electrolyte, in which a salt is dissolved in a non-aqueous organic solvent, has been mainly used as an electrolyte for an electrochemical device, such as a typical battery using an electrochemical reaction and an electric double-layer capacitor. However, when the electrolyte in a liquid state is used, an electrode material may degrade and the organic solvent is likely to be volatilized. Also, there may be limitations in safety such as combustion due to ambient temperature and the temperature rise of the battery itself.
It is known that the solid polymer electrolyte has not been commercialized yet due to poor battery performance.
Since the gel polymer electrolyte may have excellent electrochemical safety, the thickness of the battery may be constantly maintained. Furthermore, since a contact between an electrode and the electrolyte may be excellent due to the inherent adhesion of a gel phase, a thin-film type battery may be prepared. Thus, the development of various gel polymer electrolytes is being expanded.
In the gel polymer electrolyte, since the size of lithium ions may be small, direct movement may not only be relatively easy, but also the lithium ions may easily move in the electrolyte solution due to a hopping phenomenon as illustrated in FIG. 1.
A lithium secondary battery including the gel polymer electrolyte generally uses a lithium transition metal oxide, such as LiCoO2, as a cathode active material. However, when the lithium secondary battery is used at a high voltage, metal ions may be dissolved. When the metal ions are dissolved, the metal ions may be reduced to a metallic state in an anode to block reaction sites of the anode. When the new metal is precipitated on the surface of the anode, an electrolyte solution produces a new solid electrolyte interface (SEI) layer on the surface of the metal, and thus, the electrolyte solution is continuously consumed. Also, since the thickness of the SEI layer in the anode may be continuously increased to increase resistance, life characteristics of the lithium secondary battery may be decreased.